Summary: Recent study indicates that telomerase activity is highly regulated in human lymphocyte development, differentiation, and activation. However, the mode of telomerase regulation and the functional importance of telomerase in lymphocyte function remain unclear. The first part of this project is to determine the regulation of telomerase catalytic subunit, hTERT, expression and to analyze the function of telomerase in human lymphocytes. We found that hTERT is expressed in human lymphocytes at different differentiation stages regardless the levels of telomerase activity in these cells. Furthermore, although thymocytes and germinal center (GC) B cells expressed high levels of telomerase activity, only GC B cells, not thymocytes, expressed comparable high levels of hTERT mRNA. The mRNA levels of hTERT in thymocytes were not significantly higher than in peripheral blood T cells which express low to undetectable level of telomerase. These results suggest that the transcription of TERT is a necessary, but not sufficient for telomerase activity, and that the regulation of telomerase activity is likely to be determined at protein level. To assess the functional importance of telomerase, we are currently applying a retroviral expression system to alter the expression of hTERT in human lymphocyte cell lines and primary cells. Thus, the role of telomerase in lymphocyte growth and replicative lifespan can be addressed. The second part of this project is to investigate age effects on telomerase expression and telomere length in human lymphocyte subsets. We are isolating CD4+ and CD8+ T cells, and CD19+ B cells from normal individuals aged from new- born to over 90. Basal and induced levels of telomerase activity as well as telomere length will be measured and compared between the subsets of each individual as well as among the donors. The results will provide information of whether age has effect on telomerase and telomere regulation which may serve a basis for potential clinical intervention.